This invention relates to crystalline aluminosilicates. In particular, it relates to a novel crystalline aluminosilicate and its preparation.
Synthetic crystalline aluminosilicates constitute well known materials which have heretofore been employed as selective adsorbents, supports and catalysts. In general, such crystalline materials have been grown under designated conditions of temperature and time from alkali oxide, aluminum oxide, silica and water precursors. The simplest source materials for preparing the more common crystalline aluminosilicates i.e., the type A, X and Y zeolites, are sodium aluminate, sodium silicate and for the more siliceous X and Y types, an additional source of silicate ions. (The terms crystalline aluminosilicates and zeolites are used herein interchangeably and refer to the same crystalline materials.) Most of the synthesis procedures now in use are tailored to the specific zeolites being prepared.
Crystalline zeolites occur in nature and these natural materials were utilized in early investigation of their crystalline structure. Barrer, one of the early investigators, carried out the synthesis of numerous crystalline aluminosilicates using hydrothermal techniques which comprised introducing the reactant mixture into an autoclave and maintaining the mixture for extended periods of time at elevated temperatures as high as 400.degree.-450.degree. C. Subsequent investigation by others, such as Milton, accelerated the efforts for the commercial production of synthetic crystalline aluminosilicates.
Type A, Type X and Type Y zeolites are among the most useful synthetic crystalline aluminosilicates in use today. The Type A zeolite finds use in gas drying and in one of its particularly preferred embodiments, in an industrial process for separating normal paraffins from hydrocarbon mixtures. The catalytic properties of the Type X and Type Y zeolites have resulted in their commercial use in a variety of industries. By compositing the Type X or Type Y zeolite with an amorphous siliceous matrix, a catalyst is produced which has been found particularly useful in the fluid catalytic cracking of petroleum hydrocarbons.
The composition of typical crystalline zeolites is:
______________________________________ TYPE OF ZEOLITE CHEMICAL ANALYSIS ______________________________________ A Na.sub.96 [(AlO.sub.2).sub.96 (SiO.sub.2).sub.96 ] . 216H.sub.2 O X Na.sub.86 [AlO.sub.2).sub.86 (SiO.sub.2).sub.106 ] . 264H.sub.2 O Y Na.sub.56 [(AlO.sub.2).sub.56 (SiO.sub.2 O.sub.136 ] . 250H.sub.2 O ______________________________________
Breck and Flanigan in their paper "Synthesis and Properties of Union Carbide Zeolites L, X and Y" present a correlation between the lattice constant of zeolite X and zeolite Y and the number of aluminum atoms in the unit cell. (This paper was read at the conference on molecular sieves held at the University of London on Apr. 4-6, 1967 and published at pages 47-60 of "Molecular Sieves" which is a collection of the papers read at this conference and published by the Society of the Chemical Industry, London, S.W. 1 in 1968). In their paper, these authors describe the unit cell as containing 192 atoms of silicon and aluminum and define the zeolite Y structure as containing less than about 76 aluminum atoms per unit cell and the zeolite X structure as having from about 77 to 96 aluminum atoms. Extrapolation of this correlation shows a lattice constant of 25.02 A, for a zeolite having a 1:1 Si/Al ratio, i.e., 96 aluminum atoms.
Dempsey, Kuhl and Olson, J. Phys. Chem. 73, 387 (1969) present a correlation somewhat different from that of Breck and Flanigan. Their correlation between aluminum contact of the zeolite and lattice constant shows discontinuities in the correlation at specific compositions which they attribute to a high degree of ordering in the lattice at these points. Extrapolation of their data shows a lattice constant of 25.13 A for a 1:1 Si/Al atomic ratio, but the highest lattice constant of a pure zeolite actually actually measured by them was 24.996 corresponding to an SiO.sub.2 :Al.sub.2 O.sub.3 ratio of 2.45. All the tabulated zeolite have a Si:Al atom ratio of above one. The lowest ratio tabulated is (105.8/86.2 or) 1.225; and they refer to the unobtained 1:1 ratio as idealized.
In neither article do the authors present examples for aluminum content per unit cell above 87 atoms. In Breck and Flanigen the upper limit of the data shows a lattice content of 24.95 A. at 86.5 aluminum atoms while Dempsey, Kuhl and Olson show a lattice content of 24.99 A. for 86.2 aluminum atoms. The difference in lattice constant are explained by the different methods of measurement employed.
In these articles there is an implied limit of unit cell composition at a 1:1 ratio of Si to Al atoms because of the so-called "Avoidance Rule" of Lowenstein which says that in aluminosilicate-type structures aluminum ions do not occupy adjacent positions in the lattice. This then limits a Si-Al structure to a 1:1 atomic ratio since a lower ratio would require the aluminum ions to occupy adjacent positions.
A number of procedures have been described for producing synthetic zeolites. These processes require the preparation of aqueous mixtures containing oxides of sodium, silicon and aluminum within certain definite mole ratio limitations. Each process requires that this aqueous mixture be subjected to certain conditions to effect crystallization of the desired zeolite species. My U.S. Pat. No. 2,847,280 discloses the aging of an aqueous mixture of the oxides having certain definite mole ratios at a temperature not above 100.degree. F. (37.8.degree. C.) for at least 8 hours and then hydrotreating the mixture under autogenous pressure at a temperature of 150.degree.-325.degree. F. (65.6.degree.-162.8.degree. C.) to produce zeolite A. U.S. Pat. No. 2,822,243 of Milton discloses another process of producing zeolite A wherein an aqueous mixture of the oxides in specific mole ratios is maintained at 20.degree.-175.degree. C. until crystallization occurs. U.S. Pat. No. 2,982,612 of Barrer et al. discloses still another process of producing zeolite A by maintaining a certain aqueous mixture of oxides at 60.degree.-100.degree. C. to effect crystallization.
Other aqueous oxide mixes produce zeolite X or Y under proper conditions. U.S. Pat. No. 2,882,244 of Milton teaches that certain aqueous oxide mixes will produce zeolite X by maintaining the mix at 20.degree.-110.degree. C. while the mixtures disclosed in U.S. Pat. No. 2,979,381 to Gottstine et al produce the same zeolite species when aged at ambient temperature for at least two hours and maintained at 185.degree.-250.degree. F. (85.degree.-121.1.degree. C.) for at least 11/2 hours. The more siliceous aqueous mixtures of oxides disclosed in U.S. Pat. No. 3,130,007 of Breck yield zeolite Y when the mix is digested at ambient temperature for 24-32 hours and then maintained at 90.degree.-105.degree. C. for 25-65 hours.
The aging (digesting) and hydrothermal steps employed in the prior art to prepare synthetic zeolite A, X and Y were conducted under atmospheric or autogenous pressure. In no event were pressures in excess of about 35-50 psig (about 0.2-0.3 MPa) considered as being either necessary or useful in preparing these materials. Neither were the effects of high pressures investigated for any beneficial results which might be obtained when preparing these crystalline aluminosilicates.
New processes for preparing crystalline aluminosilicates including those employing high pressure, might offer significant advantages over processes presently employed, particularly if the crystalline aluminosilicate prepared thereby would have useful properties not found in crystalline aluminosilicates prepared heretofore.